The present invention relates generally to thermal spraying, and more particularly, to a method and assembly for masking a workpiece during thermal spraying to protect a portion of the workpiece from over spray.
Components are sometimes repaired using a thermal spray process to build up surfaces. Without taking steps to avoid over spray, the thermal spray also coats surfaces adjacent the surface intended to be coated. Not only can the coating degrade component performance, but processes used to remove over spray such as grit blasting can also degrade the component performance. For example, FIG. 1 illustrates a circular rabbet surface 10 of a high pressure turbine forward air seal, generally designated by 12, having a radius 14 measured from a centerline 16 of the seal. If the rabbet surface 10 is machined to a radius 14 less than engineering specifications, it may be repaired by applying a high velocity oxy fuel (HVOF) thermal spray. However, a fillet 18 immediately adjacent the rabbet surface 10 is highly stressed and cannot withstand degradation associated with over spray. Accordingly, the fillet 18 must be masked to protect it from over spray while the rabbet surface 10 is coated with thermal spray.
Although masking tapes may be used with lower velocity air plasma spray coating processes, these tapes cannot withstand the forces exerted by the HVOF thermal spray process. High temperature silicone putties designed for HVOF coating processes must be trimmed so they are positioned accurately enough to allow the coating to build up the rabbet surface 10 without allowing coating in the fillet 18. However, the trimming process may damage the component. Although thick one-piece metal masks (i.e., masks having a thickness greater than 0.1 inch) are often used to mask during HVOF thermal spray, components such as the forward air seal 12 have shapes which prevent the use of these masks because the masks obstruct the thermal spray from reaching the rabbet surface 10.
Among the several features of the present invention may be noted the provision of a mask assembly for protecting a portion of a workpiece from over spray while coating a preselected surface of the workpiece with thermal spray. The mask assembly comprises a sheet sized and shaped for covering the portion of the workpiece which the assembly is intended to protect and a support plate selectively mountable over the sheet while the surface is coated with thermal spray. Further, the mask assembly comprises a clamp mountable on the support plate for selectively attaching the support plate to the workpiece thereby clamping the support plate and sheet in position over the portion of the workpiece.
In another aspect, the present invention includes a method of masking a workpiece to protect a portion of the workpiece from over spray while coating a preselected surface of the workpiece with thermal spray. The method comprises the steps of selecting a sheet sized and shaped for covering the portion of the workpiece which the assembly is intended to protect and aligning the sheet with the portion of the workpiece so that an end of the sheet defines a boundary of the preselected surface of the workpiece. In addition, the method includes clamping the sheet to the portion of the workpiece to prevent the sheet from moving while coating the surface of the workpiece with thermal spray.
In yet another aspect, the present invention includes a mask assembly for protecting a portion of a workpiece from over spray while coating a preselected surface of the workpiece with thermal spray. The mask assembly comprises a sheet having a thickness of less than about 0.10 inch. The sheet is sized and shaped for covering the portion of the workpiece. Further, the mask assembly comprises a clamp for selectively clamping the sheet to the workpiece in position over the portion of the workpiece.
Other features of the present invention will be in part apparent and in part pointed out hereinafter.